1. Field of the Invention
The present invention relates to a nonvolatile memory device having a floating gate and to a process for manufacture of the same, more particular relates to a nonvolatile memory device designed to improve the charge retention characteristic and a process for manufacture of the same.
2. Description of the Related Art
In a nonvolatile memory device and other semiconductor devices such as erasable and programmable read only memories (EPROMs) or electrically erasable and programmable read only memories (EEPROMs) having floating gates and control gates, the electrons are retained in the floating gate so that data will not disappear even if the power is cut after data is written. The electrons injected in the floating gate, however, gradually leak to the control gate or the semiconductor substrate in accordance with a heat discharge model.
Therefore, effort has been made to prevent the leakage of electrons injected in the floating gate so as to improve the charge retention characteristics by making the interlayer insulating layer laid between the floating gate and the control gate from an ONO film (SiO.sub.2 /SiN/SiO.sub.2) superior in insulating and film-forming characteristics.
The leakage through the interlayer insulating layer can be reduced by making the interlayer insulating layer from an ONO film etc., but there is still a problem with the leakage from the side portion of the floating gate. To prevent this, the side portions of the floating gate and the control gate are sometimes covered with a side wall insulating film (referred to simply as a "side wall" here). This is to prevent the electrons injected in the floating gate from leaking in the direction of the control gate.
In the related art, the side wall is produced as follows: A floating gate and control gate are formed on the semiconductor substrate in a predetermined pattern, then a silicon dioxide film is formed on the surface of the semiconductor substrate by chemical vapor deposition (CVD). Next, the silicon dioxide film is anisotropically etched to form a side wall comprised of silicon dioxide at the side portions of the floating gate and the control gate.
However, when a nonvolatile memory device of this conventional structure was tested under high temperatures, the present inventors found that charge leakage still occurred from the floating gate. This charge leakage means a decline in the data retention rate in the case of a nonvolatile memory device. If this deteriorates, the result is a defective device.